Examining Tree Root Mitigation Techniques to Increase Tree Resiliency
Posted on Apr. 23, 2024 / Arboriculture / Subscribe 0
By: Dr. Kaufman and Myles Ritchie
Overview/The Problem
Urban forests are essential green infrastructure components in urban areas, providing environmental, ecological, social, economic, and human health benefits. In Hawaii, especially in dense urban areas like Honolulu, trees provide tremendous benefits to the approximately 9 million visitors a year along with its permanent population of around 1.4 million. On Oahu alone, approximately 235,000 trees are managed by the City and County (C&C) of Honolulu, with roughly 142,000 street trees (60%) on C&C roads (City and County of Honolulu, 2007). Forty-five percent (45%) of the C&C’s street tree budget is spent on infrastructure damage to sidewalks, gutters, roads, and sewer pipes due to damage by tree roots, amounting to approximately $2.5 million (City and County of Honolulu, 2007). Although, these same 142,000 street trees provide $2.98 in benefits for every $1 spent (City and County of Honolulu, 2007), have an estimated annual benefits valuation of $90 per tree (Department of Land and Natural Resources, 2016), amounting to cumulative annual benefits of nearly $13 million. However, even with these known benefits, urban trees are often planted in less-than-ideal areas comprised of limited root space, utility conflicts, compacted soils, insufficient nutrients, impervious surfaces, and inadequate maintenance. To further complicate matters, few studies have conducted long-term studies examining the impact of different root barrier technologies, particularly in a tropical environment such as Hawaii. The purpose of this study was to examine the efficacy of Root Barriers; Root Paths; and Silva Cell Technologies to see if one outperformed the others. The intent is to provide recommendations that could be used to mitigate the known conflicts between urban tree roots and infrastructure. From this, the goal is to have landscapes that are designed, installed and maintained using industry best practices, which is essential for a healthy urban forest.
Root Barrier Options
This first long-term tree root study in Hawaii compared root barriers, root paths, and Silva Cells tree root mitigation technologies over nearly 9 years. Root barriers are commonly used on trees in urban areas and are designed to encourage tree roots to grow downwards, reducing their occurrence in undesirable areas (sidewalks and streets) through the use of physical or chemical processes (Morgenroth, 2008). Root paths provide narrow channels of non-compacted soil, usually underneath hardscape areas, allowing tree roots to grow without obstruction from tree pits to areas with greater volumes of soil. The intent is that the roots will follow these channels, rather than causing significant infrastructure damage in the vicinity of the tree. The third root mitigation technique examined was the use of Silva Cells, which is a type of suspended pavement technology designed to guide roots into an enclosed pit/area of soil.
Methods and Materials
This study investigated the above and below ground biomass impacts of root barrier, root path, and Silva Cell technologies using 3x3 ft pit sizes for all three technologies, along with 4x4 ft and 5x5 ft pit sizes for the root barrier technology, producing a total of five treatments. The University of Hawaii, Waimanalo Research station served as the location for the study from 2014-2022 and saw the aforementioned five treatments applied to 50 trees, 25 Cassia x nealiae (Rainbow Shower) and 25 Cordia subcordata (Kou). These two species were selected due to their frequent use as urban trees in Hawaii.
Figure 1. Rainbow Shower and Kou trees at the University of Hawaii at Manoa, Waimanalo Research Station.
Each of the 50 trees had a concrete pad installed on their southwest facing side to simulate the effects of a sidewalk in an urban area. The side opposite of this concrete pad was left in its natural state to serve as a control for the treatments. Each tree had their above and below ground biomass evaluated. Above ground biomass calculations included height, diameter, live crown ratio, canopy spread, health, canopy color, tree vigor, and structural quality, with the same researcher collecting these metrics each data collection session to ensure a standardized process. Below ground biomass measurements involved excavating a pit 3 feet out from the center of each trunk to a depth of 15 inches for both the treatment (concrete pad) and control (natural state) sides of the tree. An AirSpade 2000 with a 225 cfpm nozzle and 260 cfm air compressor was used to excavate each pit.
Figure 2. Excavation of a Rainbow Shower tree using an AirSpade 2000.
The number of “concrete damaging” tree roots (diameter of >15mm) contained within a 2x1 ft quadrat was measured for each tree.
Figure 3. Measuring the number of “concrete damaging” roots.
Figure 4. Left: Root growing through root barrier treatment. Right: Root growing over root barrier treatment.
Data Analysis
The results from the five treatments (root barrier 3x3, 4x4, 5x5, root path 3x3, and Silva Cell 3x3) were compared using a one-way ANOVA test (p-value = 0.05) to determine if any statistically significant differences existed between the treatments.
Above Ground Biomass
First, the 3x3 Silva Cell treatment outperformed the 3x3 and 5x5 root barrier treatments for canopy color in the first half of the study. Second, reduced values for height, DBH, and canopy color were seen between treatments for the Rainbow Shower trees. Specifically, the 5x5 root barrier treatment had lower height values compared to all other treatments, while lower DBH values were seen with the 5x5 root barrier compared to the 3x3 root path and 3x3 and 4x4 root barrier. Finally, there were no significant differences between treatments for the Kou trees.
Below Ground Biomass
No statistical significance was seen between treatments based on the number of “concrete damaging roots” measured in the quadrat area for each tree. However, there were notable observations outside of the quadrat area when looking at the interactions between the root systems and five treatments. The major finding from these visual observations outside of the quadrat measurement area was that the Silva Cell treatment had the most centralized root masses compared to all other treatments. Furthermore, the root path treatment appeared to not be as effective as the Silva Cell treatment, but was still better than no root mitigation at all. Root barriers on the other hand were not effective when looking at the overall tree root growth patterns. This was primarily due to roots going above, below, or through the barriers to ultimately reach the surface.
Figure 5. Left: Roots growing along root path. Right:Roots growing within Silva Cell.
Discussion
The findings from this study show that the effect of root barriers on mean root depth is inconclusive, which is in line with what other research studies have shown (Morgenroth, 2018), although there are significant trends. The variation seen in the number of “concrete damaging” roots suggests that tree roots may be impacted by tree species, barrier type, and/or soil type (Morgenroth, 2018). This study also highlights issues pertaining to root barriers, particularly that roots grow through, under, and over the barrier and then upwards toward the surface (Gilman, 2006; University of Florida, 2020).
Potential Factors Affecting the Study
Three factors may have impacted the results of the study. First, a substantial flood took place at the research site in 2016 causing ground subsidence, although, the impact from this was relatively uniform across the site. Second, the quadrat method used did not provide a comprehensive depiction of the interactions that took place between all of the surrounding tree roots and mitigation techniques. The quadrat method was selected due to time and financial limitations; however, future studies should consider excavating the entirety of each tree within a 5-foot radius of the trunk to a depth of at least 24 inches to provide greater insight. Finally, as there was a relatively small sample size for each treatment (10 trees), this study should be replicated to further verify our findings.
Conclusions
The Silva Cell treatment in this study displayed the most promise to reduce infrastructure damage caused by tree roots. The root path treatment was less effective than the Silva Cell treatment, but still better than no mitigation at all. The root barrier treatment on the other hand was ineffective in containing roots in all pit sizes when viewed from outside of the quadrat area. Furthermore, smaller planting pit sizes showed greater tree vigor than the 5x5 ft pits. These larger 5x5 ft planting pits also corresponded with lower DBH values. Future research should replicate this study with the recommended methodological changes to further verify our findings. Additionally, studies should examine why smaller pit sizes produce greater values for DBH and tree vigor.
References:
City of Honolulu. (2007). Hawai‘i Municipal Forest Resource Analysis, November 2007, Center for Urban Forest Research USDA Forest Service, Pacific Southwest Research Station.
Department of Land and Natural Resources. (2016). Hawaii Forest Action Plan.
Gilman, E. F. (2006). Deflecting Roots Near Sidewalks. Arboriculture and Urban Forestry 32(1), 18-23.
Morgenroth, J. (2008). A review of root barrier research. Arboriculture and Urban Forestry. 34(2), 84-88.
University of Florida. (2020). Roots Growing Under Barrier-Barrier Removed, Landscape Plants.
Dr. Kaufman is an Associate Professor and Landscape Specialist for the University of Hawaii. His educational background consists of a Bachelor’s degree in Ornamental Horticulture from Cal Poly San Luis Obispo; Master’s degree in Landscape Architecture from the University of Arizona; Master’s degree in Sociology from Iowa State University; and a Ph.D. in Horticulture from
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